Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 338,886	$ 218,074
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	4,837	972
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	86,047	28,931
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	1,878	403
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	424,933	247,005
Held-to-maturity securities, Gross Unrealized Losses, Total	6,715	1,375
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	248,415	135,576
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	3,474	520
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	91,515	76,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	4,199	2,194
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	339,930	211,661
Available-for Sale, Gross Unrealized Losses, Total	7,673	2,714
Fair Value of Investments with Unrealized Losses, Less Than 12 months	587,301	353,650
Gross Unrealized Loss, Less Than 12 Months	8,311	1,492
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	177,562	105,016
Gross Unrealized Losses, 12 Months or Greater	6,077	2,597
Fair Value of Investments with Unrealized Losses, Total	764,863	458,666
Gross Unrealized Losses, Total	14,388	4,089
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	237,284	110,698
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	3,737	654
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	35,373	19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	1,036	301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	272,657	130,593
Held-to-maturity securities, Gross Unrealized Losses, Total	4,773	955
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	97,971	23,365
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2,118	86
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	21,048	19,153
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,736	491
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	119,019	42,518
Available-for Sale, Gross Unrealized Losses, Total	3,854	577
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	67,454	56,425
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	584	156
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	21,100	4,121
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	295	11
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	88,554	60,546
Held-to-maturity securities, Gross Unrealized Losses, Total	879	167
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	133,712	82,795
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	772	171
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	9,615	5,888
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	157	41
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	143,327	88,683
Available-for Sale, Gross Unrealized Losses, Total	929	212
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	1,962	2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	65	40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	482
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	22
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,444	2,798
Held-to-maturity securities, Gross Unrealized Losses, Total	87	40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	8,356	14,686
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	288	59
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,424	3,328
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	188	740
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	12,780	18,014
Available-for Sale, Gross Unrealized Losses, Total	476	799
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	31,676	48,153
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	443	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	29,054	3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	523	76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	60,730	52,101
Held-to-maturity securities, Gross Unrealized Losses, Total	966	198
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	8,376	14,730
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	296	204
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	56,428	47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,118	922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	64,804	62,446
Available-for Sale, Gross Unrealized Losses, Total	2,414	1,126
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	510
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	8
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	38	967
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	2	15
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	548	967
Held-to-maturity securities, Gross Unrealized Losses, Total	$ 10	$ 15